Population cannot be transfered from one colony to another. POP migration is a concept we'll possibly explore in one of the expansions. It will probably be more about population slowly migrating from one place to another instead of actually picking a POP from one colony and putting it on another.
To boost POP in new colonies you can allocate production to Habitat Control, put a Support Ship in orbit, build a Cloning Facility or research POP growth techs.
Guess I missed this somewhere, but...
How do you transfer colonists from one (perhaps crowded world) to one that could, apparently, use some extra people?
